Propellers are well known for many uses which provide, for given applications, preselected fluid flow and efficiency characteristics. In certain processes, particularly those operating upon liquids or suspensions within a containing vessel, such as mixing, conditioning, agitation and attrition scrubbing, it is beneficial to provide a flow pattern substantially parallel to the axis of rotation of the propeller and shaft, particularly in the local region of the propeller. Propellers are often difficult to manufacture, and many attempts have been made to improve propellers in terms of manufacturing techniques, material usage and operational power requirements.
For example, U.S. Pat. No. 745,853 discloses a propeller having blades with faces curved in a complex manner to impart to water within which it turns a motion parallel to the shaft axis. The blades are shaped as a true cycloidal curve wherein plural angles of lead of pitch vary as a function of distance from the propeller hub. For maximum strength, the blades are affixed to a hub at a 45.degree. angle, and are thicker at the hub than at the outer ends. This configuration is said to minimize the presence of surfaces or parts which serve as a drag to impede motion of a vessel driven by the propeller, and may appear to be useful in a mixing environment. The blades, however, are difficult to manufacture as a result, among other considerations, of the complex cycloidal shape, and thus may not be efficiently suitable for use in processes taking place within a containing vessel.
Another teaching, U.S. Pat. No. 1,444,923, discloses a blade for a fan useful where strength characteristics in response to fluid induced forces is not as critical, such as in the movement of air. The blades, therefore, are formed of a composite material, hardened with a binder. The materials are initially molded into sections of hollow cylinders, or other regular shapes such as curved sheets, strips or hemispheres, and the blades are subsequently punched from the curved sections at an angle to the axis of the sections. The blades are mounted on a hub in a manner such that the central longitudinal dimension, or so called axis of the punched blade, extends radially from the hub. While blades so constructed may alleviate excessive labor and expense relative to other fabrication techniques, there is no indication that a propeller formed in this manner will provide a flow of air substantially parallel to the axis of hub rotation.
Another teaching, U.S. Pat. No. 4,147,437, discloses an impeller for imparting turbulent motion to a fluid medium within a containing vessel, and particularly shows blades useful in adjusting the degree of angular dispersion of the medium from the axis of impeller rotation. The impeller is said to be efficient in terms of power consumption. Trapezoidal sheets are curved, such as by cylindrical circular rolling, in a manner such that the central axis of the symmetrical trapezoidal sheet is at a preselected angle to the rollers and thus also to the surface of the curved sheet. Where the preselected angle is zero, it is stated that a conical flow pattern, from the impeller toward the vertical sides of the containing vessel, of approximately 45.degree. is obtained. Where the preselected angle is positive as defined, approximately 20.degree., a cylindrical flow pattern, that is, flow parallel to the axis of rotation of the impeller, is obtained. Similar flow patterns are said to be obtained through compound, specifically angled and shaped blade surfaces and flaps. While desired flow patterns within containing vessels can be obtained, manufacturing of such impellers is relatively complex requiring, among other steps, fabrication of the trapezoidal sheet and rolling or pressing at a specific angle. The latter operations are made more difficult where thick blades are desired.
It is thus desirable to provide propellers and blades which are relatively simple to fabricate and which provide a fluid flow pattern generally parallel to the axis of rotation of the propeller. It is further desirable to provide such propellers which operate with low power consumption to achieve a desired fluid motion or degree of turbulence.